Display laminate with single packet biaxially birefringent reflective polarizer

ABSTRACT

Multilayer optical film reflective polarizers previously considered to have excessive off-axis color can provide adequate performance in an LC display without any high haze light diffusing layer or air gap between the reflective polarizer and the back absorbing polarizer of the display. The reflective polarizer has only one packet of microlayers, and is oriented using a standard tenter such that birefringent microlayers in the film are biaxially birefringent. The microlayers in the packet have a layer thickness profile suitably tailored to avoid excessive perceived color at normal and oblique angles. A laminate made by combining this type of reflective polarizer with an absorbing polarizer, without an air gap or any high haze light diffusing layer or structure between the polarizers, can be used and incorporated into a liquid crystal display or the like with adequate color performance both at normal incidence and oblique incidence up to a polar angle of 60 degrees.

FIELD OF THE INVENTION

This invention relates generally to reflective polarizing films, withparticular application to such films that form a laminate with at leastan absorbing polarizer. The invention also relates to associatedarticles, systems, and methods.

BACKGROUND

Reflective polarizers are commonly used to enhance the brightness ofliquid crystal (LC) displays and display systems. The LC display systemtypically includes an LC panel, behind which is an illumination assemblyor backlight positioned to provide light to the LC panel. Brightnessenhancement is provided by the reflective polarizer as the result of alight recycling process: light that cannot (because of its polarizationstate) contribute to the display output is reflected by the reflectivepolarizer back into the backlight, where some of the light isre-reflected towards the reflective polarizer in a differentpolarization state that can contribute to the display output and thatpasses through the reflective polarizer toward the user or viewer.

The LC panel includes a layer of liquid crystal material disposedbetween glass panel plates. Furthermore, the LC panel is sandwichedbetween two absorbing polarizer films: a front absorbing polarizer,attached to the front glass plate of the LC panel, and a back absorbingpolarizer, attached to the back glass plate. The brightness-enhancingreflective polarizer is placed somewhere behind the LC panel, and behindthe back absorbing polarizer.

In practice, design details of the reflective polarizer have an impacton exactly where the reflective polarizer can be placed in the displaysystem to provide optimal, or at least acceptable, optical performance.Some types of reflective polarizers can be laminated directly to theexposed rear surface of the back absorbing polarizer. Those of ordinaryskill in the art consider it necessary for these types of reflectivepolarizers to have a very low perceived color for the pass state ofpolarization both at normal incidence (light propagating along theoptical axis of the display system) and at highly oblique incidence.Since the reflective polarizer is attached to the back absorbingpolarizer, and the back absorbing polarizer is in turn commonly attachedto the back glass plate of the LC panel, this is referred to as an“on-glass” configuration of the reflective polarizer. One reflectivepolarizer currently used in the on-glass configuration is aparabolically-stretched reflective polarizer, discussed further below.Another reflective polarizer used in the on-glass configuration is amulti-packet reflective polarizer, also discussed below.

Other types of reflective polarizers, considered by those of ordinaryskill in the art to have excessive perceived color for the pass state ofpolarization for obliquely incident light, are not laminated to the backabsorbing polarizer of the display because the (undesirable) colorassociated with the reflective polarizer would be visible to the userthrough the absorbing polarizer and through the LC display. Instead,these latter types of reflective polarizers are used in the displaysystem as a stand-alone film, separated from the back absorbingpolarizer by at least one air gap, and attached to a light diffusingfilm or layer that is disposed between the reflective polarizer and theback absorbing polarizer. The light diffusing layer has a significanthaze value so as to effectively combine light rays that pass through thereflective polarizer in different directions, to reduce or eliminate thecolor associated with the reflective polarizer from the standpoint ofthe user or viewer.

BRIEF SUMMARY

Broadly speaking, the stand-alone and on-glass configurations havecertain practical advantages and disadvantages relative to each other.One advantage of the stand-alone reflective polarizer, separated as itis from the back absorbing polarizer and from the LC panel by an airgap, is that the reflective polarizer is mechanically decoupled from theLC panel, thus reducing or eliminating mechanical interactions betweenthe reflective polarizer and the LC panel. These interactions mayinclude, for example: panel-induced bending or wrinkling of thereflective polarizer at elevated environmental test conditions; andlamination defects, or defects in the reflective polarizer film, whichmay require rework of the LC panel. A disadvantage of the on-glassconfiguration is the relatively high cost of the reflective polarizer,which in some cases is due to the low yield of parabolically-stretchedreflective polarizers (discussed below), and in other cases is due tothe high material cost of multiple packet reflective polarizer products(also discussed below). However, a benefit of the on-glass configurationis the simplicity and convenience of having a single polarizer productthat includes both the back absorbing polarizer and the reflectivepolarizer in one laminate film product.

We have found that certain multilayer optical film reflective polarizerspreviously considered to have too much off-axis color to be used in anon-glass configuration—hence, previously considered to be limited in adisplay system to a stand-alone configuration with an attached high hazelight diffusing layer and spaced apart from the back absorbing polarizerby an air gap to reduce unwanted color effects—can actually provideacceptable performance in an LC display without the need for any suchair gap or high haze light diffusing layer. Thus, a laminate made bycombining such a reflective polarizer with an absorbing polarizer,without an air gap and without a high haze light diffusing layer orstructure (and in some cases without any significant light diffusinglayer or structure) between the reflective polarizer and the absorbingpolarizer, can be successfully used and incorporated into a liquidcrystal display or the like. The reflective polarizer in thisconstruction is a multilayer optical film of alternating polymer layersin which there is only one packet of microlayers, the multilayer opticalfilm having been oriented using a standard tenter such that birefringentlayers of the film are biaxially birefringent. The microlayers in thepacket are provided with a layer thickness profile appropriatelytailored to avoid excessive perceived color at normal and obliqueangles. Such multilayer optical film reflective polarizers are discussedfurther below.

We thus describe herein, inter alia, optical film laminates that includea reflective polarizer and an absorbing polarizer. The reflectivepolarizer has only one packet of microlayers that reflects and transmitslight by optical interference, the microlayers configured to define afirst pass axis, a first block axis, and a first thickness axisperpendicular to the first pass axis and the first block axis. Theabsorbing polarizer has a second pass axis and a second block axis, andis attached to the reflective polarizer with no air gap therebetween,the first and second pass axes being substantially aligned. The packetof microlayers includes alternating first and second microlayers, and atleast the first microlayers are biaxially birefringent. The reflectivepolarizer in isolation is characterized by a spectral transmission, forp-polarized light incident at a polar angle of 60 degrees in a referenceplane that includes the first pass axis and the first thickness axis, inwhich (a) the spectral transmission has a value in a range from 70% to90%, or from 70% to 85%, for at least some wavelengths from 450 to 700nm; and (b) a high frequency spectral variability Δ (Greek letterdelta), over a wavelength range from 400 to 700 nm, is less than 0.08,or less than 0.05.

Related methods, systems, and articles are also discussed.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side or sectional view of a liquid crystal displaysystem;

FIG. 2 is a schematic side or sectional view of a single packetmultilayer optical film configured as a reflective polarizer;

FIG. 3 is a perspective view of a web of optical film;

FIG. 4 is a perspective view of an optical film or laminate in relationto a Cartesian coordinate system;

FIG. 5 is a graph of spectral transmission to illustrate the concept ofa best-fit curve, which can be used to calculate a high frequencyspectral variability parameter Δ;

FIG. 6 is a schematic perspective view of a multilayer optical filmreflective polarizer disposed behind and spaced apart from an absorbingpolarizer, the reflective polarizer provided with a light diffusinglayer to reduce the amount of observed color;

FIG. 7 is a schematic perspective view of a laminate of a multilayeroptical film reflective polarizer and an absorbing polarizer, with nolight diffusing layer;

FIG. 8 is a schematic perspective view of a laminate similar to that ofFIG. 7 but further including a glass layer from a liquid crystal panel,the absorbing polarizer being disposed between the reflective polarizerand the glass layer;

FIG. 9 is a graph of the spectral transmission at normal incidence forfour multilayer optical film reflective polarizer samples, for lightpolarized along the block axis, each sample being obtained from acentral portion of its respective film web;

FIG. 10 is a graph similar to that of FIG. 9 for the four reflectivepolarizer samples, but for normally incident light polarized along thepass axis;

FIG. 11 is a graph similar to that of FIG. 10 for the four reflectivepolarizer samples, but where the light is incident at a 60 degree polarangle in a plane that includes the pass axis and the normal axis, thelight being p-polarized in such plane of incidence;

FIG. 12 is a graph similar to that of FIG. 11, for four reflectivepolarizer samples taken from a first edge portion of their respectivefilm webs but otherwise corresponding respectively to the fourreflective polarizer samples of FIG. 11;

FIG. 13 is a graph similar to that of FIGS. 11 and 12, for fourreflective polarizer samples taken from a second edge portion of theirrespective film webs but otherwise corresponding respectively to thefour reflective polarizer samples of FIGS. 11 and 12; and

FIG. 14 is a graph of the spectral transmission for one of thereflective polarizer samples from FIG. 11, and the spectral transmissionof that reflective polarizer sample laminated to an absorbing polarizer,in both cases for light incident at a 60 degree polar angle in a planethat includes the pass axis and the normal axis, the light beingp-polarized in such plane of incidence.

In the figures, like reference numerals designate like elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As mentioned above, we have found that certain types of multilayeroptical film reflective polarizers previously considered to haveexcessive off-axis color can actually provide adequate performance in aliquid crystal display without the need for any air gap or high hazelight diffusing layer (and in some cases with no light diffusing layeror structure at all) between the reflective polarizer and the backabsorbing polarizer of the LC display. The type of reflective polarizerat issue has only one packet of microlayers, and is oriented using astandard tenter such that birefringent microlayers in the film arebiaxially birefringent. Furthermore, the microlayers in the singlepacket have a suitably tailored layer thickness profile. Previously, itwas believed this type of reflective polarizer needed to have a highhaze light diffusing layer, and needed to be separated from the rearabsorbing polarizer by an air gap, to avoid unwanted color effects fromthe standpoint of the user or viewer of the display. We have found thata laminate made by combining this type of reflective polarizer with anabsorbing polarizer, without an air gap or any high haze light diffusinglayer or structure between the polarizers, can be used and incorporatedinto a liquid crystal display or the like with adequate colorperformance both at normal incidence and oblique incidence, up to apolar angle of at least 60 degrees.

In general, if one is given a multilayer optical film reflectivepolarizer of unspecified design, appropriate placement of thatreflective polarizer in an LC display system is a function of, amongother things, color characteristics of the reflective polarizer,particularly color characteristics at high off-axis (oblique) angles oflight propagation. Such color characteristics are, in turn, a functionof the manner in which the film was fabricated, and the film's resultingphysical and optical features.

For example, it is known to fabricate a reflective polarizer bycoextruding tens, hundreds, or thousands of alternating polymer layersthrough a die, optionally doubling or tripling the number of layers bysplitting and re-stacking the flow stream in a layer multiplier device,cooling the extrudate on a casting wheel, and orienting (stretching) thecast film to reduce the film thickness such that individual polymerlayers form optically thin microlayers, and to induce birefringence inat least some of the microlayers. In the finished multilayer opticalfilm, the microlayers reflect and transmit light by opticalinterference, as a function of refractive index differences betweenadjacent microlayers, optical thicknesses of adjacent pairs ofmicrolayers, and the thickness profile of a stack of such layer pairsalong a thickness direction or axis of the film. To make a reflectivepolarizer, the orientation or stretching is carried out primarily alongone in-plane direction, so that the refractive indices of themicrolayers define a block axis of high reflectivity, a pass axis of lowreflectivity (and high transmission), and a thickness axis perpendicularto the pass and block axes. See for example U.S. Pat. No. 5,882,774(Jonza et al.).

FIG. 1 is provided for reference to illustrate various components,layers, and films that may be included in a typical LC display system100. The display system 100 includes a display panel 150 and anillumination assembly 101 positioned behind the panel 150 to providelight thereto. The display panel 150 can include any suitable type ofdisplay. In the illustrated embodiment, the display panel 150 includes,or is, an LC panel (hereafter referred to as LC panel 150). The LC panel150 typically includes a layer of liquid crystal (LC) 152 disposedbetween panel plates 154 a, 154 b (collectively, 154). The plates 154are often composed of glass and can include electrode structures andalignment layers on their inner surfaces for controlling the orientationof the liquid crystals in the LC layer 152. These electrode structuresare commonly arranged so as to define LC panel pixels, i.e., areas ofthe LC layer where the orientation of the liquid crystals can becontrolled independently of adjacent areas. A color filter may also beincluded with one or more of the plates 152 for imposing desired colorssuch as red, green, and blue on subpixel elements of the LC layer, andthus on the image displayed by the LC panel 150.

The LC panel 150 is positioned between a front (or upper) absorbingpolarizer 156 and a back (or lower) absorbing polarizer 158. In theillustrated embodiment, the front and back absorbing polarizers 156, 158are located outside the LC panel 150. Often, the absorbing polarizer(156 or 158) is laminated to the outer major surface of its neighboringglass panel plate (154 a or 154 b respectively) with a suitabletransparent adhesive. The absorbing polarizers 156, 158 and the LC panel150 in combination control the transmission of light from a backlight110 through the display system 100 to the viewer. For example, theabsorbing polarizers 156, 158 may be arranged with their pass axes(transmission axes) perpendicular to each other. Selective activation ofdifferent pixels of the LC layer 152, e.g. by a controller 104, resultsin light passing out of the display system 100 at certain desiredlocations, thus forming an image seen by the viewer. The controller 104may include, for example, a computer or a television controller thatreceives and displays television images.

One or more optional layers 157 may be provided proximate the frontabsorbing polarizer 156, for example, to provide mechanical and/orenvironmental protection to the display surface. The layer 157 may forexample include a hardcoat over the front absorbing polarizer 156.

The illumination assembly 101 includes a backlight 110 and one or morelight management films in an arrangement 140 positioned between thebacklight 110 and the LC panel 150. The backlight 110 can be or includeany known backlight of suitable design. For example, light source(s)within the backlight may be positioned such that the backlight is of theedge-lit variety or the direct-lit variety.

The arrangement 140 of light management films, which may also bereferred to as a light management unit, is positioned between thebacklight 110 and the LC panel 150. The light management films affectthe illumination light propagating from the backlight 110. In some casesthe backlight 110 can be considered to include one, some, or all of thelight management films in the arrangement 140.

The arrangement 140 of light management films may include a diffuser148. The diffuser 148 is used to scatter or diffuse the light receivedfrom the backlight 110. The diffuser 148 may be any suitable diffuserfilm or plate. For example, the diffuser 148 can include any suitablediffusing material or materials. In some embodiments, the diffuser 148may include a polymeric matrix of polymethyl methacrylate (PMMA) with avariety of dispersed phases that include glass, polystyrene beads, andCaCO₃ particles. The diffuser 148 may also be or include 3M™ Scotchcal™Diffuser Film, types 3635-30, 3635-70, and 3635-100, available from 3MCompany, St. Paul, Minn., USA. A diffuser 148 as used in a lightmanagement film arrangement such as arrangement 140 would typically havea relatively high haze, e.g. at least 40%, as measured using a HazeGuard Plus haze meter from BYK-Gardiner, Silver Springs, Md., accordingto a suitable procedure such as that described in ASTM D1003.

The light management unit 140 also includes a reflective polarizer 142.Although in a general sense the reflective polarizer 142 may be of anysuitable design—for example, a multilayer optical film, a diffuselyreflective polarizing film (DRPF) such as a continuous/disperse phasepolarizer, a wire grid reflective polarizer, or a cholesteric reflectivepolarizer—for purposes of the present application we are interested incases where the reflective polarizer is a particular type of multilayeroptical film, as discussed elsewhere herein. For example, the reflectivepolarizer may be a multilayer optical film of alternating polymer layersin which there is only one packet of microlayers, the multilayer opticalfilm having been oriented using a standard tenter such that birefringentlayers (including birefringent microlayers) of the film are biaxiallybirefringent. Those of ordinary skill in the art have regarded this typeof reflective polarizer as having so much off-axis color that a highhaze diffuser and air gap between the reflective polarizer 142 and theback absorbing polarizer 158 was considered necessary to keep theoverall perceived color of the display system 100 at or reasonably neara neutral white color.

In some embodiments, a polarization control layer 144, such as a quarterwave retarding layer, may be provided between the diffuser 148 and thereflective polarizer 142. The polarization control layer 144 may be usedto change the polarization of light that is reflected from thereflective polarizer 142 so that an increased fraction of the recycledlight is transmitted through the reflective polarizer 142.

The arrangement 140 of light management films may also include one ormore brightness enhancing layers. A brightness enhancing layer canredirect off-axis light in a direction closer to the axis of thedisplay. This increases the amount of light propagating on-axis throughthe LC layer 152, thus increasing the brightness of the image seen bythe viewer. One example of a brightness enhancing layer is a prismaticbrightness enhancing layer, which has a number of prismatic ridges thatredirect the illumination light through refraction and reflection. InFIG. 1, a first prismatic brightness enhancing layer 146 a providesoptical gain in one dimension, and a second prismatic brightnessenhancing layer 146 b has prismatic structures oriented orthogonally tothose of layer 146 a, such that the combination of layers 146 a, 146 bincreases the optical gain of the display system 100 in two orthogonaldimensions. In some embodiments, the brightness enhancing layers 146 a,146 b may be positioned between the backlight 110 and the reflectivepolarizer 142.

The different layers in the light management unit 140 may be freestanding relative to each other. Alternatively, two or more of thelayers in the light management unit 140 may be laminated to each other.

Two design aspects of the multilayer optical film reflective polarizerto be used in the LC display system are of particular relevance to thepresent application: the manner in which the extruded film isstretched—which in practical effect determines whether the birefringentmicrolayers are uniaxially birefringent or biaxially birefringent—, andwhether layer multiplier devices are used during fabrication, or whetherthe finished multilayer optical film has more than one distinct stack ofmicrolayers.

We first discuss the manner of stretching or orienting the extrudedfilm. In a first known technique, a long length or web of polymer filmcontinuously advances through a standard tenter apparatus. In thestandard tenter, the film is held tautly by sets of clips attached toopposite edges of the film, and the clip sets move forward along rails,under the action of a chain drive or the like. In one section of thetenter, straight sections of the rails diverge from each other such thatthe clips stretch the film in the cross-web direction (also called thetransverse direction) as the clips carry the film generally forward inthe down-web direction (also called the longitudinal direction). Thisorients the film primarily in the cross-web direction. The clips in thestandard tenter maintain a constant clip-to-clip spacing and move at aconstant speed throughout the length of the straight rail sections,which prevents the film from relaxing in the down-web direction. Due tothis down-web constraint of the film during orientation, the stretchprovided by such a standard tenter is sometimes referred to as aconstrained stretch. As a consequence of the constraint, layers withinthe film that become birefringent under the conditions of the stretchtypically develop three different refractive indices along the threeprincipal directions (the cross-web or x-direction, the down-web ory-direction, and the thickness or z-direction) of the film. If we denotethe refractive indices of such a layer along the principal x-, y-, andz-directions as nx, ny, and nz, then nx≠ny, and ny≠nz, and nz≠nx. (Tothe extent the material exhibits dispersion, whereby a given refractiveindex n changes somewhat as a function of optical wavelengths, therefractive index may be understood to be specified at a particularvisible wavelength such as 550 nm (green) or 632.8 nm (He—Ne laser,red), or the refractive index may be understood to be an average overthe visible wavelength range, e.g. from 400-700 nm.) A material havingthis type of birefringence is said to be biaxially birefringent.

In a reflective polarizer in which birefringent microlayers alternatewith isotropic microlayers, a consequence of the birefringentmicrolayers being biaxially birefringent is that the layer-to-layerrefractive index differences along the y-direction and along thez-direction cannot both be zero. This in turn results in residualreflectivity and (when used in a display) perceived color for light thatpropagates at high oblique angles relative to an optical axisperpendicular to the film, for p-polarized light propagating in areference plane that includes the y-axis (i.e., the pass axis of thepolarizer) and the z-axis.

In a second known technique, the film or web advances through astretching apparatus that has been specially designed to allow the webor film to fully relax in the down-web direction during the orientationprocess. For example, in some embodiments the stretching apparatusutilizes sets of clips that move along parabolically-shaped rails. Seee.g. U.S. Pat. No. 6,949,212 (Merrill et al.). By allowing the film torelax in the down-web direction (as well as in the thickness direction),layers within the film that become birefringent under the conditions ofthe stretch typically develop only two different refractive indicesalong the three principal directions of the film. Stated differently,for such a birefringent layer, the refractive index along thez-direction equals, or substantially equals, the refractive index alongthe y-direction, but those refractive indices differ substantially fromthe refractive index along the x-direction (the direction of stretch).Using the nx, ny, nz notation, ny=nz, but nx≠ny, and nx≠nz. (In somecases ny and nz may not be exactly equal, but their difference is verysmall, as discussed below. Thus, ny≈nz.) A material having this type ofbirefringence is said to be uniaxially birefringent. In a reflectivepolarizer in which birefringent microlayers alternate with isotropicmicrolayers, a consequence of the birefringent microlayers beinguniaxially birefringent is that the layer-to-layer refractive indexdifferences along the y-direction and along the z-direction can both bemade to be zero, or substantially zero, while the refractive indexdifference along the x-direction is nonzero and large in magnitude. Thisresults in little or no significant reflectivity at high oblique angles,and little or no perceived color at such angles when the film is used asa reflective polarizer in a display.

Thus, with regard to off-axis color in a display, a multilayerreflective polarizer whose birefringent microlayers are uniaxiallybirefringent, e.g. made using a parabolic stretching apparatus, has aninherent advantage relative to a polarizer whose birefringentmicrolayers are biaxially birefringent, e.g. made using a conventionaltenter. However, in practice, with all other factors being equal, auniaxially birefringent polarizer is more costly to manufacture than abiaxially birefringent polarizer, at least in part due to substantiallylower yields for the specialized parabolic stretching apparatus comparedto those for the standard tenter.

Optical materials that may be used in the fabrication of the disclosedreflective polarizers can be selected from known materials, preferablytransparent polymer materials whose material properties allow for thecoextrusion of such materials at the same temperature and in a commonfeedblock. In exemplary embodiments, layers of alternating thermoplasticpolymers (ABABAB . . . ) are used, and one of the polymers is selectedto become birefringent, and the other polymer is selected to remainoptically isotropic, under the conditions of stretching. Suitablepolymers may be judiciously selected from, for example, polyethylenenaphthalate (PEN), polyethylene terephthalate (PET), polybutyleneterephthalate (PBT), copolymers thereof, and blends thereof.Additionally, other classes of polymers that exhibit birefringence andmay be useful for this purpose are polystyrenes (including syndiotacticpolystyrene), polyamides (including Nylon6), and liquid crystalpolymers.

Regarding the above discussion relating to uniaxial and biaxialbirefringence, and equalities and inequalities involving nx, ny, and nz,we recognize that exact equality between two refractive indices may bedifficult to achieve or measure, and, from a practical standpoint, smalldifferences may be indistinguishable from exact equality. Therefore, forpurposes of this document, we consider a material to be uniaxiallybirefringent if one pair of its refractive indices are substantially thesame, e.g., if they differ by less than 0.05, while remaining pairs ofits refractive indices are not substantially the same, e.g., if theydiffer by at least 0.05. Likewise, a material is considered to bebiaxially birefringent if each and every pair of its principalrefractive indices are not substantially the same, e.g., if they differby at least 0.05.

Typically, with regard specifically to multilayer optical filmreflective polarizers, a biaxially birefringent layer in such polarizermay for example have refractive indices nx, ny, nz that satisfy therelationships |ny−nz|≥0.05, and |nx−ny|>0.06 or 0.08. In contrast, auniaxially birefringent layer in such polarizer may for example haverefractive indices nx, ny, nz that satisfy the relationships|ny−nz|<0.05, and |nx−ny|>0.06 or 0.08.

Another design aspect of particular relevance to the present applicationis the number of distinct stacks of microlayers that are present in thefinished multilayer reflective polarizer, which is often related towhether layer multiplier devices were used during fabrication of thefilm. In describing this feature, reference is made to FIG. 2, whichschematically depicts a single packet multilayer optical film configuredas a reflective polarizer 220.

The multilayer optical film or polarizer 220 has two opposed outer majorsurfaces 220 a, 220 b, between which are a plurality of distinct polymerlayers. Polymer materials and film-making equipment that can be used tomake such a film by coextrusion and stretching are known, see e.g. U.S.Pat. No. 5,882,774 (Jonza et al.) and U.S. Pat. No. 6,783,349 (Neavin etal.), and patent application publication US 2011/0102891 (Derks et al.).Adjacent polymer layers have substantially different refractive indicesalong at least one of the principal x, y, or z axes, so that some light(depending upon the direction of propagation and the polarization stateof the light) is reflected at interfaces between the layers. Some of thepolymer layers of the polarizer 220 are sufficiently thin (“opticallythin”) so that light reflected at a plurality of the interfacesundergoes constructive or destructive interference in order to give themultilayer optical film the desired reflective or transmissiveproperties. These layers are referred to herein as microlayers, and arelabeled “A” and “B” in FIG. 2. For reflective polarizers designed toreflect visible light, each microlayer generally has an opticalthickness (i.e, a physical thickness multiplied by its refractive index)of less than about 1 micron. Thicker layers, such as skin layers orprotective boundary layers (PBLs), as are known in the art, may also bepresent in the polarizer, as illustrated in FIG. 2 by the layer 222.Such “optically thick” layers have an optical thickness of at least 1micron, and often much greater than 1 micron, and are not considered tobe microlayers. (Throughout this document, when the term “thickness” isused without the modifier “optical”, the thickness refers to thephysical thickness, unless otherwise indicated by the context.)

A coherent grouping of microlayers is referred to herein as a packet ofmicrolayers, or microlayer packet. As shown, the polarizer 220 containsonly one packet 224 of microlayers. The packet 224 has a (physical)thickness of T₁, and the polarizer 220 has an overall thickness of T₂,as shown in the figure. Configuring the multilayer optical film withonly one packet 224 of microlayers simplifies the manufacturing process(provided the number of microlayers desired is not excessive) and allowsfor greater control of the thicknesses and thickness profiles of themicrolayers, which in turn allows for greater control of the spectralreflectivity and spectral transmission characteristics of the reflectivepolarizer. In FIG. 2, pairs of adjacent microlayers form optical repeatunits (ORUs), labeled ORU1 through ORU6, each ORU having an opticalthickness (OT1, OT2, . . . OT6) equal to the sum of the opticalthicknesses of its constituent microlayers. Although only 6 ORUs (12microlayers) are shown, the reader will understand that a typical singlepacket reflective polarizer will contain many more microlayers and ORUsin order to provide adequate reflectivity over the visible spectrum. Forexample, the total number of microlayers in the single packet reflectivepolarizer may be less than 500, or less than 400, or less than 350, orin a range from 200 to 500, or from 200 to 400, or from 200 to 350, orfrom 225 to 325, for example. The optical thickness of an ORU determinesthe wavelength at which the ORU exhibits peak reflectivity. Carefulcontrol of the thicknesses of the ORUs in accordance with a desiredlayer thickness profile, wherein the optical thicknesses of the ORUsgradually increase from one side of the packet (e.g. near the majorsurface 220 a) to the opposite side of the packet (e.g. near the thicklayer 222), allows the packet of microlayers to provide a broadreflectivity over the visible spectrum and over a desired range ofobservation angles, provided a sufficient number of ORUs are present inthe packet.

An alternative approach, to more easily achieve the desired opticalperformance targets, is to design the multilayer optical film reflectivepolarizer to have more microlayers than can be practically incorporatedinto a single packet film. For this reason (or for other reasons),reflective polarizers are made in which the microlayers are divided orseparated into two or more distinguishable microlayer packets, with atleast one optically thick polymer material separating neighboringpackets. Such multiple packet reflective polarizers can be manufacturedin various ways. For example, the reflective polarizer can be made usingmultiple feedblocks (corresponding to the multiple packets) andcombining the packets from these feedblocks while the polymer materialsare still liquid, rather than using only one feedblock. See e.g. patentapplication publication US 2011/272849 (Neavin et al.). Alternatively,the reflective polarizer can be made using a layer multiplier device,e.g. as discussed in U.S. Pat. No. 5,882,774 (Jonza et al.) or U.S. Pat.No. 6,025,897 (Weber et al.). The layer multiplier device may forexample double or triple the number of microlayers and ORUs, producingtwo or three times (respectively) the number of packets in the finishedreflective polarizer. In still another approach, a multiple packetreflective polarizer can be made by laminating together two or moremultilayer optical film reflective polarizers that were each made with,for example, a single feedblock.

Drawbacks of multiple packet reflective polarizers tend to include (a)increased manufacturing cost because of the large number of layers andresulting high material cost, and (b) relatively large overall physicalthickness, which can be a significant disadvantage in some displayapplications. (The disclosed reflective polarizers desirably have athickness of less than 50 microns, or less than 40 microns, or in arange from 25 to 50 microns or from 25 to 40 microns.) However, thelarger number of microlayers allow the multiple packet reflectivepolarizers to achieve display-quality optical performance targets evenwhen such polarizers are oriented using a standard tenter, that is, evenwhen the birefringent microlayers in such reflective polarizers arebiaxially birefringent. This is because the multiple packets can producea spectral smoothing as described in patent application publication US2013/0063818 (Weber et al.), resulting in a reduced amount of off-axiscolor. Single packet reflective polarizers cannot take advantage of thisspectral smoothing technique, and have a smaller margin of error withrespect to layer thickness variability.

When discussing multilayer optical films made by coextruding numerouslayers of alternating polymer materials through a feedblock/die andorienting the film with a stretching operation, and the suitability ofsuch films in visual display applications, one aspect of the film ofpractical interest to the person of ordinary skill is the degree towhich the as-manufactured film is spatially uniform. This aspect of thefilm is of interest because it relates to how much of the manufacturedfilm can be used, versus how much must be disposed of, in the intendedapplication. This in turn affects the manufacturing yield and cost ofmanufacture, and it can also place size limitations on how large of apiece can be obtained or cut from a given web of film to fit a largedisplay system. In the case of optical films for use in LC displays, ahigh degree of spatial uniformity is desirable so that film-relatedartifacts are not noticeable in the displayed image.

A web of optical film 320 is shown schematically in FIG. 3. The film 320is manufactured on a film-making line and emerges from a tenter or otherstretching device, which is depicted schematically as element 309. Thefilm 320 has a longitudinal or down-web direction parallel to they-axis, as shown. The film 320 also has a transverse or cross-webdirection parallel to the x-axis, as shown. Two opposed longitudinaledges 320 a, 320 b define the longitudinal boundaries of the film 320.It is near these edges that the clip sets from the tenter or specializedstretching apparatus grasped the film during a previous orientationstep, after which the film 320 was trimmed to the edges 320 a, 320 b.Three film samples, intended for use as reflective polarizers in adisplay application or other desired application, are shown in thefigure: a film sample 321 a near the film edge 320 a, a film sample 321b near the film edge 320 b, and a film sample 321 c in a central portion(in relation to the transverse direction) of the film 320. These filmsamples or pieces are cut from the larger web or film 320 with a knife,slitter, or other suitable cutting implement. As a reflective polarizer,the optical film 320, and each of the film samples 321 a, 321 b, 321 c,has a block axis parallel to the x-axis, and a pass axis parallel to they-axis.

In an idealized situation, the film samples 321 a, 321 b, 321 c will allhave the same optical characteristics and properties. However, inpractice, the film 320 exhibits a certain amount of spatial variability.As a result, the layer thickness profile of the microlayer packet (andits corresponding spectral transmission and reflection characteristics)near an edge of the film 320 differs somewhat from the layer thicknessprofile (and corresponding spectral transmission and reflectioncharacteristics) in the central portion of the film. The amount ofchange in the spectral characteristics between the center and edge ofthe film is particularly significant for the type of multilayer opticalfilm of interest to the present application, namely, a reflectivepolarizer having only one packet of microlayers and oriented using astandard tenter such that birefringent microlayers in the film arebiaxially birefringent. The significant spatial variability contributesto the belief by others that these films would not be acceptable for usein an on-glass configuration as described above.

Elsewhere in this document we discuss optical properties such astransmission and reflection of certain polarizing films and laminates atspecific angles and polarization states. FIG. 4 is provided to assistthe reader in understanding the relevant directions, planes, and angles.In the figure, an optical body 412, which may for example be or comprisea multilayer optical film configured as a reflective polarizer, or sucha film laminated to an absorbing polarizer and/or to another opticalfilm or body, is shown in the context of a Cartesian x-y-z coordinatesystem. As a polarizer, the optical body 412 has a pass axis 413 and ablock axis 414, which correspond to the mutually perpendicular y- andx-axes, respectively. The z-axis corresponds to a thickness direction ofthe body 412, i.e., an axis perpendicular to the plane of the body 412.Light that is normally incident on the body 412 propagates parallel tothe z-axis. Such light is substantially transmitted by the body 412 ifthe light has a linear polarization component parallel to the pass axis413, and is substantially blocked (reflected in the case of a reflectivepolarizer, absorbed in the case of an absorbing polarizer) if the lighthas a linear polarization component parallel to the block axis 414.

For lack of an alternative term, “plane of incidence” is used herein torefer to the reference plane containing the surface normal direction andthe light propagation direction, both in cases where the light isincident on the film, and in cases where light is not incident on thefilm but instead is emerging from the film. Likewise, “incidence angle”is used to refer to the angle between the surface normal direction andthe light propagation direction, both for light incident on the film andfor light emerging from the film.

Two reference planes of incidence, 416 and 418, are included in thefigure: reference plane 416 contains the block axis 414 and the z-axis;and reference plane 418 contains the pass axis 413 and the z-axis. Twoobliquely incident light rays 415, 417 are shown in the figure. Ray 415lies in plane 416, and ray 417 lies in plane 418. The rays 415, 417 areobliquely incident because their directions of propagation formrespective non-zero polar angles θ with respect to the z-axis. For eachray 415, 417, the polarization state of the light ray can be resolvedinto two orthogonal components, represented in the figure as a pair oforthogonal double-headed arrows: a component whose polarization state isin the plane of incidence, referred to as “p-polarized”, and a componentwhose polarization state is perpendicular to the plane of incidence,referred to as “s-polarized”. Inspection of the figure reveals that thepolarization direction of p-polarized light for oblique ray 415 is notthe same as (and is not parallel to) the polarization direction ofp-polarized light for oblique ray 417. Similarly, the polarizationdirection of s-polarized light for oblique ray 415 is not the same as(and is not parallel to) the polarization direction of s-polarized lightfor oblique ray 417. Also apparent is that the p-polarized (“p-pol”)component of ray 415 is perpendicular to the pass axis 413 and partiallyaligned with the block axis 414, while the s-polarized (“s-pol”)component of ray 415 is parallel to the pass axis 413. The p-polcomponent of ray 417 is perpendicular to the block axis 414 andpartially aligned with the pass axis 413, while the s-pol component ofray 417 is parallel to the block axis 414. From this, one can see thatdepending on the direction of incidence, p-polarized light can beperpendicular to the pass axis in some cases and perpendicular to theblock axis in others, and s-polarized light can be parallel to the passaxis in some cases and parallel to the block axis in others.

One way of quantifying the color associated with a given transmission(or reflection) spectrum involves calculating the high frequencyspectral variability of the spectrum over a range of visiblewavelengths. We refer to the calculated high frequency spectralvariability as Δ, i.e., the Greek letter delta. Curves that helpillustrate this approach are shown in the transmission versus wavelengthgraph of FIG. 5. In the graph, curve 501 is an arbitrary curve oftransmission versus wavelength for a given polarizer in a givenillumination geometry. The curve 501 exhibits some variability over thevisible wavelength range, which we may assume to extend from 400 to 700nm, although other reasonable endpoint wavelengths may instead be used.The variability of the curve 501 is manifested to a user or viewer as acolored (non-white) appearance when the film is illuminated with whitelight. Of particular interest is any color that is due to relativelyhigh frequency variability as a function of wavelength. For example,spectral variability having a period of roughly 20 nm or less can resultin a viewed color that changes rapidly, both spatially and angularly.

To quantify the high frequency variability Δ, and thus the colorassociated with such variability, we first define a smoothed spectrumwith which to compare the curve 501. The smoothed spectrum should be abest-fit curve to the curve 501, e.g., from a least-squares or weightedleast-squares sense, and the smoothed spectrum should be a lowmathematical order polynomial so that only the higher mathematicalorders—corresponding to higher frequency variability—are included in thecomparison. Preferably, the smoothed spectrum is a best-fit curve ofthird order in wavelength, i.e., a third order polynomial as follows:

a ₀ +a ₁ λ+a ₂λ² +a ₃λ³,

where the coefficients a₀, a₁, a₂, and a₃ are selected according to aleast-squares methodology. Such a best-fit third order spectrum wascalculated for the curve 501, the result being curve 502. The actualtransmission (curve 501) can then be compared to the best-fit spectrum(curve 502) by subtracting curve 502 from curve 501, the result being acomparison curve 503. Note that the comparison curve 503 is notphysically meaningful insofar as it contains values of transmission thatare negative. Nevertheless, the curve 503 can be used to derive aphysically meaningful value for the high frequency variability contentof the curve 501. To do so, we calculate the statistical standarddeviation of the comparison curve 503 over the same visible spectrum,e.g. from 400 to 700 nm. For the curve 503 illustrated in FIG. 5, thisstandard deviation equals 9.18%, or 0.0918. The high frequencyvariability Δ of the curve 501 can thus be said to be 9.18%, or 0.0918.The units of this high frequency variability number are the same as theunits of the curves 501, 502, and 503. Thus, if the units of the curves501, etc. are given as percent transmission (0% minimum, 100% maximum),then the units of the high frequency variability are also percenttransmission, or simply percent. If instead the units of the curves 501,etc. are given as simply the transmission (0.0 minimum, 1.0 maximum),then the units of the high frequency variability Δ are also simplytransmission, with no percent.

Turning now to FIG. 6, we see there schematically illustrated selectedelements of an LC display system 600. The selected elements shown are aback absorbing polarizer 658 (which may be the same as or similar to theback absorbing polarizer 158 in FIG. 1), a multilayer optical filmreflective polarizer 620 (which may be the same as or similar to thereflective polarizer 142 of FIG. 1, or the reflective polarizer 220 ofFIG. 2), and a light diffusing layer 625 disposed on the front majorsurface of the reflective polarizer 620. Other components that would beincluded in the LC display system, such as an LC panel, a frontabsorbing polarizer, and a backlight, are omitted from the figure forsimplicity. The optical films lie generally in, or parallel to, the x-yplane. A first user or viewer 608 is located in front of the system 600and views the display at normal incidence, along a system optical axisparallel to the z-axis. A second user or viewer 609 is also located infront of the system 600, but views the display at an oblique angle.

The back absorbing polarizer 658 is assumed to be any of the absorbingpolarizers known in the art for their suitability in LC displays. Thepolarizer 658 has a pass axis and a block axis (not shown in FIG. 6),the polarizer being oriented such that the pass axis is parallel to they-axis, and the block axis is parallel to the x-axis.

The reflective polarizer 620 is assumed to be a multilayer optical filmof alternating polymer layers in which there is only one packet ofmicrolayers. It is further assumed that the polarizer 620 has beenoriented using a standard tenter, such that birefringent layers of thepolarizer 620 are biaxially birefringent. FIG. 6 depicts the polarizer620 in a stand-alone configuration, in keeping with the belief by othersthat this type of reflective polarizer 620 is not suitable for anon-glass configuration due to excessive off-axis color of the polarizer620. Thus, the reflective polarizer 620 is separated from the absorbingpolarizer 658 by an air gap 605. Furthermore, the reflective polarizer620 is provided with a light diffusing layer 625 on one major surfacethereof, the diffusing layer 625 being disposed between the reflectivepolarizer 620 and the absorbing polarizer 658. The light diffusing layer625 scatters light into a cone or distribution of angles as shown byincident light ray 606 and scattered light rays 607. The scatteringeffectively mixes light rays that propagate through the reflectivepolarizer 620 in different directions to reduce or eliminate colorassociated with the reflective polarizer 620. The diffusing layer 625 isassumed to have a relatively high haze, e.g. at least 40%, as measuredusing a Haze Guard Plus haze meter. The diffusing layer 625 may be ofany known type or design, for example, it may comprise glass or ceramicbeads or other particles immersed in a matrix of a different refractiveindex, or it may comprise a textured, faceted, or otherwise non-smoothmajor surface at a polymer/air or polymer/polymer interface.

As already mentioned, we have found through investigation and testingthat, contrary to prevailing opinion, a single-packet, biaxiallybirefringent reflective polarizer can provide acceptable opticalperformance in an on-glass configuration, i.e., when laminated to a backabsorbing polarizer (with no diffusing layer or structure therebetween,although in some cases such a layer or structure may be included thathas a relatively low haze, e.g., less than 30%, or less than 20%, orless than 10% haze). Two examples of an on-glass configuration are shownin FIGS. 7 and 8.

In the schematic view of FIG. 7, a laminate 730 or optical body isshown, wherein a multilayer optical film reflective polarizer 720attaches to a back absorbing polarizer 758 by a transparent adhesivelayer 726. The reflective polarizer 720, the back absorbing polarizer758, and the adhesive layer 726 are all coextensive with each other, andthere is no air gap between the reflective polarizer 720 and theabsorbing polarizer 758. A viewer side of the laminate 730 is in thepositive z direction, thus, the back absorbing polarizer 758 may beconsidered to be in front of the reflective polarizer 720. Thereflective polarizer 720 may be the same as or similar to the reflectivepolarizer 620 described above. Thus, the reflective polarizer 720 is asingle packet multilayer optical film made by coextrusion of alternatingpolymer layers, the film having birefringent microlayers that arebiaxially birefringent as a result of the polarizer 720 having beensubjected to a constrained stretch on a conventional tenter. Thereflective polarizer 720 may be a central portion of a reflectivepolarizer web, see e.g. film sample 321 c in FIG. 3, or it may be anedge portion, see e.g. film samples 321 a, 321 b.

The reflective polarizer 720 has a pass axis 713 a, generally parallelto the y-axis, and a block axis 714 a, generally parallel to the x-axis.The number of ORUs in the single microlayer packet, and the thicknessprofile of those ORUs, provides the reflective polarizer 720 with a hightransmission for normally incident visible light polarized parallel tothe pass axis 713 a, and a low transmission (and high reflection, sincetransmission+reflection is about equal to 100% for these low-absorptionmultilayer optical films) for normally incident visible light polarizedparallel to the block axis 714 a. For example, the transmission ofnormally incident visible light polarized parallel to the pass axis 713a may be at least 60%, or at least 70%, or at least 80% when averagedover the visible wavelength range, and the transmission of normallyincident visible light polarized parallel to the block axis 714 a may beless than 30%, or less than 20%, or less than 10%, when averaged overthe visible wavelength range. Optical performance of the reflectivepolarizer 720 for oblique p-polarized light, incident in a referenceplane that contains the z-axis and the pass axis 713 a, is influenced bythe unavoidable layer-to-layer refractive index mismatches resultingfrom the biaxially birefringent nature of the birefringent microlayersin the film. For such oblique light at a 60 degree polar angle ofincidence, the transmission of the reflective polarizer 720 has a valuein a range from 70% to 90%, or from 70% to 85%, for at least somewavelengths from 450 to 700 nm; in some cases, the transmission for suchoblique light may be less than 90% throughout a wavelength range from400 to 500 nm.

The reflective polarizer 720 may have an overall thickness of less than50 microns, or less than 40 microns, or it may be in a range from 20 to50 microns, or in a range from 20 to 40 microns, or in a range from 25to 40 microns. The layer thickness profile of the ORUs in the microlayerpacket of the polarizer 720 may be tailored so that the high frequencyspectral variability Δ for the transmission of the polarizer for theoblique 60 degree p-polarized light is less than 0.08 (i.e., less than8%), or less than 0.05 (i.e., less than 5%), as calculated over awavelength range from 400 to 700 nm relative to a best fit curve that isa third order polynomial with wavelength.

The back absorbing polarizer 758, which has a pass axis 713 b and ablock axis 714 b, may be the same as or similar to the back absorbingpolarizer 658 described above. The absorbing polarizer 758 is orientedrelative to the reflective polarizer 720 such that the pass axes 713 a,713 b are substantially aligned, and the block axes 714 a, 714 b arealso substantially aligned. For example, two such substantially alignedaxes may be characterized by an angular deviation of less than 1 degree,or less than 0.1 degrees.

The transparent adhesive layer 726 may be any suitable optical adhesive,for example, any of the Optically Clear Adhesive products available from3M Company, St. Paul, Minn. The refractive index of the adhesive layer726 is desirably reasonably close to the refractive index of theabsorbing polarizer 758 and the refractive indices of the reflectivepolarizer 720, to avoid Fresnel reflection at the polymer/adhesiveinterfaces of those films. The adhesive layer 726 preferably provides apermanent bond between the absorbing polarizer 758 and the reflectivepolarizer 720.

The laminate 730 may consist (only) of, or it may consist essentiallyof, the reflective polarizer 720, the absorbing polarizer 758, and theadhesive layer 726. In some embodiments, the laminate 730, and each ofthese three components, does not incorporate any significantidentifiable light diffusing layer or structure, such as beads or otherparticles of different refractive index, or a textured or othernon-smooth major surface. The laminate 730 may thus be devoid of anysuch light diffusing layer or structure. However, in cases where thelaminate 730 does include such a diffusing layer or structure, it isoften desirable to at least ensure that no such diffusing layer orstructure is disposed between the reflective polarizer 720 and theabsorbing polarizer 758. The foregoing statements are made with therecognition that even ideal, flat optical films and layers withexceptional optical clarity may exhibit a minute but measureable amountof optical scattering or diffusion. Thus, for clarity, we may establisha minimal threshold below which the layer or structure at issue may beconsidered, from a practical standpoint and for the purposes of thepresent document, to have no light diffusion. We set this minimal lightdiffusion threshold at a haze value of 5%, or 4%, or 3%, or 2%, or 1%,as measured using a Haze Guard Plus haze meter from BYK-Gardiner, SilverSprings, Md., according to a suitable procedure such as that describedin ASTM D1003.

Optical films are often sold and/or shipped with a temporary polymericrelease liner on both sides to protect the major surfaces of the filmfrom scratches or other damage. Such release liners can be easilyremoved from the product by peeling. The release liners can incorporatedyes, pigments, or other agents, including light diffusing agents, sothey can be easily seen or detected by the user. Such temporary releaseliners may be applied to the outer surfaces of the laminate 730 as well.However, such release liners are distinguishable from, and need not beconsidered part of, the laminate 730. Thus, to the extent such releaseliners are present on the laminate 730 (or on other laminates disclosedherein, including laminate 830 below) and have a substantial lightdiffusion property, it can still be correct to state that the laminatedoes not incorporate any significant light diffusing layer or structure.

The reader should be cautioned, however, that in some cases it can bedesirable to include one or more moderate diffusing layers or structuresbetween the reflective polarizer 720 and the absorbing polarizer 758,such moderate diffusing layers or structures having an amount of hazethat is significant, i.e., greater than the above-mentioned minimallight diffusion threshold, yet smaller than high haze diffuserstypically found in stand-alone configurations such as that of FIG. 6. Adiffusing layer or structure may for example be included between thereflective polarizer 720 and the absorbing polarizer 758 that has arelatively low haze, e.g., less than 30%, or less than 20%, or less than10% haze.

The layer thickness profile used in the disclosed biaxially birefringentreflective polarizers warrants some additional discussion. As alreadymentioned, the microlayers in the microlayer packet are organized intooptical repeat units (ORUs), and the optical thicknesses of the ORUs(and microlayers) are tailored to provide, for light throughout thevisible spectrum, a high broadband reflectivity for light of the blockpolarization, and a high broadband transmission (low reflectivity) forlight of the pass polarization, over a desired range of incidence anglesand directions. This is typically accomplished by tailoring thethickness profile of the ORUs along the thickness direction (z-axis) ofthe film to be a monotonic, or near-monotonic, function, with thinnerORUs located generally at one side of the packet (referred to here asthe thin side), and thicker ORUs located generally at the opposite sideof the packet (referred to here as the thick side). In order to reducethe undesirable perceived off-axis transmitted color of the disclosedfilms, it can be useful in at least some embodiments to (a) orient thereflective polarizer such that the thick side of the microlayer packetfaces the observer (or absorbing polarizer or LC panel), and the thinside of the microlayer packet faces the backlight, and (b) tailor theORU thickness profile to be smoothly varying so that the spectraltransmission of the film for highly oblique incident light of the passpolarization state is likewise smoothly varying over the visiblespectrum, and (c) tailor the ORU thickness profile in such a way as toavoid an excessive number of ORUs at the thick end of the packet beyondthe point at which an ORU has a resonant reflectivity peak at 650 nm atthe oblique angle limit, even though such tailoring can produce anundulation in the transmission spectrum of the reflective polarizer forpass state light incident at or near the oblique angle limit.

Another laminate 830 or optical body is shown in FIG. 8. The laminate830 may be the same as or similar to the laminate 730 as describedabove, except that two additional layers have been added. Thus, thelaminate 830 includes a back absorbing polarizer 858, a multilayeroptical film reflective polarizer 820, and an adhesive layer 826 thatbonds the absorbing polarizer 858 to the reflective polarizer 820. Theseelements may be the same as or similar to corresponding elements of thelaminate 730, and they form an optical body or structure 830′ which maythus be the same as or similar to the laminate 730, except that thefront of structure 830′ is attached to additional layers. In particular,the front major surface of the back reflective polarizer 858 is bondedto a glass layer 854 through an adhesive layer 828. The adhesive layer828 may be the same as or similar to the adhesive layer 826. The glasslayer may be the back or rear panel plate of a liquid crystal panel,such as the panel plate 154 b of the LC panel 150, described above.

The laminate 830 may consist (only) of, or it may consist essentiallyof, the elements 820, 826, 858, 828, and 854 as described above. Similarto the laminate 730, the laminate 830 and each of its componentspreferably does not incorporate any significant identifiable lightdiffusing layer or structure, such as beads or other particles ofdifferent refractive index, or a textured or other non-smooth majorsurface. The laminate 830 may thus be devoid of any such light diffusinglayer or structure. However, in cases where the laminate 830 doesinclude such a diffusing layer or structure, it is often desirable to atleast ensure that no such diffusing layer or structure is disposedbetween the reflective polarizer 820 and the absorbing polarizer 858. Asdiscussed above, even ideal, flat optical films and layers withexceptional optical clarity may exhibit measureable optical scattering,and we may establish a minimal threshold below which the layer orstructure at issue may be considered to have no light diffusion forpurposes of the present document. Suitable threshold values are givenabove. Furthermore, in some cases it can be desirable to include one ormore diffusing layers or structures between the reflective polarizer 820and the absorbing polarizer 858 that have a small but significant amountof haze, e.g., less than 30%, or less than 20%, or less than 10% haze.

EXAMPLES

Several reflective polarizers, and laminates using such polarizers, weremade and tested. All of the reflective polarizers were fabricated bycoextruding numerous layers of two alternating polymer materials througha feedblock and die, and orienting the film with a stretching operationto produce a multilayer optical film reflective polarizer. For eachfilm, one of the polymer materials became birefringent under theconditions of the stretch, while the other polymer material remainedsubstantially isotropic.

In brief, one of the reflective polarizers, “Example 1”, was made usingno layer multiplier device and had only one packet of microlayers.Further, the Example 1 polarizer was oriented using a standard tenter,such that the birefringent microlayers in the film were biaxiallybirefringent. Another reflective polarizer, “Comparative Example 1”, wassimilar in design and construction to the Example 1 polarizer, but thelayer thickness profile of the ORUs in the packet of microlayers wasless controlled than that of the Example 1 polarizer, and hence thespectral transmission over visible wavelengths was more variable.Another reflective polarizer, “Comparative Example 2”, was made using alayer multiplier device that produced three distinct microlayer packetsin the film. Similar to Example 1 and Comparative Example 1, theComparative Example 2 polarizer was oriented using a standard tenter,producing biaxially birefringent microlayers. Another reflectivepolarizer, “Comparative Example 3”, was made with only one packet ofmicrolayers, but was oriented using a parabolic stretching apparatus,producing uniaxially birefringent microlayers. Samples of thesereflective polarizers were obtained from both central portions and edgeportions of their respective film web, as explained below. The Example1, Comparative Example 2, and Comparative Example 3 reflectivepolarizers are representative of reflective polarizer products that havebeen offered for sale in the United States for more than one year.However, applicants are not aware of the Example 1 reflective polarizerever being sold or used in an on-glass configuration, e.g., in laminateform with an absorbing polarizer.

Example 1

Of the two alternating polymer materials used to make the Example 1polarizer, the birefringent polymer was a copolyester (referred to hereas 90/10 coPEN) composed of 90 mol % naphthalene dicarboxylate and 10mol % dimethyl terephthalate, with 100% of the diol being ethyleneglycol. The isotropic polymer was a blend of 58 wt % of PETg GN071 fromEastman Chemical Company, Kingsport, Tenn., and 42 wt % of the 90/10coPEN. This copolymer has an isotropic refractive index of about 1.593for 633 nm light. These two polymer materials were coextruded using a275 layer feedblock and film-making equipment similar to that describedin U.S. Pat. No. 6,783,349 (Neavin et al.), except that no layermultiplier device was used. The layer thickness profile of the 275layers (approximately 137 ORUs) was controlled to substantially match atarget monotonic optical thickness profile using an axial rod heaterdisposed in the feedblock whose temperature profile was dynamicallyadjusted along its length during coextrusion to maintain the targetlayer thickness profile with a minimum of deviation. The finishedpolarizing film, referred to herein as the Example 1 polarizer, includedan optically thick skin layer composed of PETg GN071 at both the frontand back of the microlayer packet, the skin layers forming the outermostlayers of the film exposed to air. The Example 1 polarizer was orientedin a standard tenter as described above with a draw ratio of about 6:1in the transverse direction and 1:1 in the machine direction(longitudinal direction), at a rate of about 60% per second. For thisfilm, the temperature in the pre-heat zone (where the film is heatedbefore substantial stretching occurs) was 312 degrees F., thetemperature in the stretch zone (where the film is stretched) was 287degrees F., and the temperature in the heat-set zone (where the film isstill gripped by the tenter clips after the stretch zone) was 290degrees F. In the heat-set zone, the film was relaxed 0.3% in thetransverse direction and the length was unchanged in the machinedirection.

The Example 1 polarizer had a finished film thickness of about 30microns, with each outer skin layer having a thickness of about 1micron.

Comparative Example 1

The Comparative Example 1 polarizer was made in the same manner as theExample 1 polarizer, except that: (a) the isotropic polymer was the PETgGN071 material discussed above (whose refractive index, 1.563 for 633 nmlight, is lower than that of the isotropic polymer of Example 1), and b)the temperature profile of the axial rod heater was not adjusted in thesame manner as in Example 1. This resulted in a layer thickness profileof the 275 layers (approximately 137 ORUs) that was not optimized tosubstantially match the target monotonic optical thickness profile ofExample 1, which in turn resulted in a more highly variable transmissionspectra, and more colorful appearance at oblique angles. The ComparativeExample 1 polarizing film was oriented in the standard tenter to about6:1 in the transverse direction and 1:1 in the machine (longitudinal)direction.

The Comparative Example 1 polarizer had a finished film thickness ofabout 32 microns, with each outer skin layer having a thickness of about1 micron.

Comparative Example 2

Of the two alternating polymer materials used to make the ComparativeExample 2 polarizer, the birefringent polymer was polyethylenenaphthalate (PEN). The isotropic polymer was a copolyester (referred tohere as coPEN 55/45 HD) composed of 55 mol % naphthalene dicarboxylate,and 45 mol % dimethyl terephthalate with 96 mol % of the diol beingethylene glycol and 8 mol % of the diol being hexanediol. These twopolymer materials were coextruded in an alternating layer arrangementhaving 275 total layers, and the extrudate was sent through a 3:1 layermultiplier device that divides the extrudate and stacks the threeextrudate components (packets) atop each other. The layer thicknessprofiles of the individual packets are managed by use of an axial rodheater system to avoid an excessive number of microlayers or ORUs at aparticular thickness so as to avoid having reflectivity peaks for anyparticular wavelength. The resulting cast web was further processed andstretched in a standard tenter to a draw ratio of about 6:1 in thetransverse direction and a draw ratio of 1:1 in the machine(longitudinal) direction. This produces a reflective polarizing filmwith about 825 total microlayers separated into three distinctmicrolayer packets of 275 microlayers (about 137 ORUs) each, withoptically thick protective boundary layers (PBLs), as mentioned above,therebetween, and optically thick skin layers composed of the PETg GN071material at the outer major surfaces.

The Comparative Example 2 polarizer had a finished film thickness ofabout 94 microns, with each outer skin layer having a thickness of about7 microns, and each PBL having a thickness of about 4 microns.

Comparative Example 3

Of the two alternating polymer materials used to make the ComparativeExample 3 polarizer, the birefringent polymer was the 90/10 coPEN. Theisotropic polymer was a copolymer composed of 85 wt % Xylex EXXX0282,available from SABIC located in Riyadh, Saudi Arabia, and 15 wt % PETgGN071 from Eastman Chemical Co. These two polymer materials werecoextruded in an alternating layer arrangement with 275 total layers,which became 275 microlayers (about 137 ORUs) in the single packetfinished film. The layer thickness profile of the 275 layers wascontrolled to substantially match a target monotonic optical thicknessprofile using an axial rod heater disposed in the feedblock, whosetemperature profile was dynamically adjusted along its length duringcoextrusion to maintain the target layer thickness profile with aminimum of deviation. The cast web was further processed and stretchedwith a parabolic stretching apparatus as described in U.S. Pat. No.6,949,212 (Merrill et al.), with a transverse direction draw ratio ofabout 6:1 and a machine (longitudinal) direction stretch ratio of about0.5:1 (i.e., reduced to ½ of its length in the machine direction) toproduce a finished reflective polarizing film with 275 total microlayersarranged into a single microlayer packet with optically thick skinlayers at the outer major surfaces. As described in the '212 Merrill etal. patent, for these multi-layered materials and this type ofdeformation, there is a close match in refractive index along the y- andz-directions between adjacent birefringent and isotropic microlayers, sothat there is very little reflectivity produced by the multi-layer stackfor p-pol pass state light at any angle of incidence.

Samples of these four reflective polarizer films were then taken fromdifferent places on their respective film webs, e.g. as suggested byFIG. 3, and the samples were tested. FIGS. 9 through 14 show some of thetest results.

FIG. 9 shows the measured transmission for normally incident lightpolarized parallel to the block axis of each reflective polarizersample. These measurements are for reflective polarizer film samplestaken from the central portion of each respective film web. Curve 900 isthe measured transmission for a sample of the Example 1 reflectivepolarizer. Curves 901, 902, and 903 are the measured transmission forsamples of the Comparative Example 1, Comparative Example 2, andComparative Example 3 reflective polarizers, respectively.

FIG. 10 is a graph similar to that of FIG. 9 for the same fourreflective polarizer samples, but for normally incident light polarizedparallel to the pass axis of each such polarizer. Thus, curve 1000 isthe measured transmission for the Example 1 sample, and curves 1001,1002, and 1003 are the measured transmission for samples of theComparative Example 1, Comparative Example 2, and Comparative Example 3samples, respectively.

The polarizing films were also evaluated by visual observation at normalincidence. Each of the four reflective polarizer samples from FIGS. 9and 10 (which were taken from the central portions of the respectivefilm webs) were observed at normal incidence with white lightillumination, and each of those polarizer samples exhibited little or nocolor in transmission. That is, when placing a given polarizer samplebetween a white light source and the viewer's eye, at normal incidence,the polarizer sample did not substantially change the white appearanceof the light source. This was also found to be true for samples of theExample 1 reflective polarizer, Comparative Example 1 reflectivepolarizer, Comparative Example 2 reflective polarizer, and ComparativeExample 3 reflective polarizer that were taken from edge portions ofeach respective film web.

A more difficult test of a reflective polarizer, in terms of itsperceived color, is the appearance of the reflective polarizer at asignificant oblique angle. At oblique observation angles, any refractiveindex difference between microlayers in the thickness direction(z-direction) becomes significant, and a shift in the transmissionspectrum to shorter wavelengths also occurs. FIGS. 11, 12, and 13 showthe measured transmission spectra for various samples of the four testedreflective polarizers under the following observation or illuminationgeometry: the spectral transmission of each sample was tested at a 60degree polar angle, for light in a plane of incidence (reference plane)that contains the z-axis and the y-axis (pass axis) of the film, thelight being polarized in such plane of incidence, i.e., p-polarized.(Furthermore, in each case the tested film was oriented such that thethick side of its microlayer packet(s) faced the light source of thespectrophotometer.) With this geometry, Fresnel reflections at the outerpolymer/air interfaces of the film are small or negligible.

FIG. 11 shows the 60 degree p-pol transmission spectra as describedabove for each of the reflective polarizer film samples that were usedin FIGS. 9 and 10, namely, film samples taken from the central portionof the respective film webs (see e.g. film sample 321 c in FIG. 3).Thus, curve 1100 is for such a sample from the Example 1 reflectivepolarizer, curve 1101 is for such a sample from the Comparative Example1 reflective polarizer, curve 1102 is for such a sample from theComparative Example 2 reflective polarizer, and curve 1103 is for such asample from the Comparative Example 3 reflective polarizer.

The Comparative Example 3 sample (curve 1103) shows very hightransmission with a small amount of variation due to the layer-to-layerrefractive index match in both the y- and z-directions (made possible byuniaxially birefringent microlayers), and the well-controlled layerthickness profile. The Example 1 sample (curve 1100) and the ComparativeExample 2 sample (curve 1102) show a lower transmission throughout thevisible region, which can be explained by the use of a standard tenterin the manufacture of these films, producing biaxial birefringence and asignificant layer-to-layer refractive index difference in thez-direction. However, the relatively small spectral variability for thecurves 1100 and 1102 is indicative of good layer thickness control and awell-tailored layer thickness profile of the microlayers and ORUs in themicrolayer packet(s) of these samples. The Comparative Example 1 sample(curve 1101) has a lower average transmission because of the lowerrefractive index isotropic polymer used in the film than that of theother samples, but furthermore, the curve 1101 is quite variable in thevisible wavelength region, which is indicative of a high perceivedcolor.

The curve 1100 is in a range from 70% to 85% transmission for at leastsome wavelengths from 400 to 700 nm. The curve 1100 is also less than90% throughout a wavelength range from 400 to 500 nm.

The high frequency variability Δ of the curves 1100 through 1103 wascalculated, where a third order polynomial was in each case used as thebest-fit curve, and where the calculation was performed over thewavelength range from 400 to 700 nm. The results are as follows: Δ forcurves 1100, 1101, 1102, and 1103 was 0.0118, 0.0515, 0.0216, and0.0043, respectively. These results are consistent with a visualobservation of the film samples at the 60 degree geometry: theComparative Example 3 sample exhibited the least amount of color, theExample 1 and Comparative Example 2 samples exhibited more color thanComparative Example 3 but still an amount of color that is acceptable insome display applications, and Comparative Example 1 exhibited the mostcolor. Note that Δ for the Example 1 sample (curve 1100) is less than0.08, and less than 0.05, but is at least 0.01.

To investigate the effect of spatial variability at different regions ofthe film web, measurements were also made on samples of the reflectivepolarizers taken from the edges of the respective film webs.

FIG. 12 is a graph of spectral transmission taken under the samemeasurement conditions as FIG. 11, but in FIG. 12 the film samples weretaken from a first edge portion of the respective film webs, see e.g.film sample 321 a in FIG. 3. Thus, curve 1200 is for such a sample fromthe Example 1 reflective polarizer, curve 1201 is for such a sample fromthe Comparative Example 1 reflective polarizer, curve 1202 is for such asample from the Comparative Example 2 reflective polarizer, and curve1203 is for such a sample from the Comparative Example 3 reflectivepolarizer.

FIG. 13 is a graph of spectral transmission taken under the samemeasurement conditions as FIGS. 11 and 12, but in FIG. 13 the filmsamples were taken from a second edge portion of the respective filmwebs, see e.g. film sample 321 b in FIG. 3. Thus, curve 1300 is for sucha sample from the Example 1 reflective polarizer, curve 1301 is for sucha sample from the Comparative Example 1 reflective polarizer, curve 1302is for such a sample from the Comparative Example 2 reflectivepolarizer, and curve 1303 is for such a sample from the ComparativeExample 3 reflective polarizer.

The FIG. 12 and FIG. 13 graphs follow a trend similar to FIG. 11. Thus,for the samples taken from the edges of the film webs, ComparativeExample 3 still exhibits the highest and smoothest transmission overvisible wavelengths. Comparative Example 1 still exhibits the lowestaverage transmission and the most spectral variability, and hence color.And Example 1 and Comparative Example 2 still show an intermediatetransmission, whose spectral variability in the visible region isgreater than that of Comparative Example 3 but less than that ofComparative Example 1.

The high frequency variability Δ of the curves in FIGS. 12 and 13 wascalculated in the same way as described above in connection with FIG.11. The results are as follows: Δ for curves 1200, 1201, 1202, and 1203was 0.0383, 0.0918, 0.0421, and 0.0049, respectively, and Δ for curves1300, 1301, 1302, and 1303 was 0.0516, 0.0607, 0.0339, and 0.0059,respectively. These results are consistent with a visual observation ofthese edge-vicinity film samples at the 60 degree geometry: theComparative Example 3 sample exhibited the least amount of color, theExample 1 and Comparative Example 2 samples exhibited more color thanComparative Example 3 but still an amount of color that is acceptable insome display applications, and Comparative Example 1 exhibited the mostcolor.

Laminates of the various reflective polarizer films were also made bybonding the reflective polarizer to an absorbing polarizer using atransparent optical adhesive layer, e.g. as shown schematically in FIG.7. The absorbing polarizer was HL C2-5618S from Sanritz Corporation ofTokyo, Japan. The optical adhesive was provided with the HL C2-5618Spolarizer from Sanritz. There was no air gap between the absorbingpolarizer and the reflective polarizer, and these polarizers wereoriented such that their pass axes were substantially aligned, and suchthat their block axes were also substantially aligned. No lightdiffusing layer or structure was disposed between the reflectivepolarizer and the absorbing polarizer, and no light diffusing layer wasincluded in the laminate at all. Such laminates were made both withreflective polarizer samples taken from the central portion of thereflective polarizer web, and samples taken from the edge portion of theweb.

In these laminates, the absorbing polarizer had a relatively minoreffect on the transmission spectrum that was observed with thereflective polarizer by itself, and a relatively minor effect on thevariability of such spectrum. This is demonstrated in FIG. 14. There,curve 1401 is the transmission spectrum of a sample of the ComparativeExample 1 reflective polarizer taken from a central portion of the filmweb, for p-polarized light incident in a plane that includes the z-axisand the y-axis (pass axis), at a 60 degree polar angle. A laminate asdescribed in the preceding paragraph was made with this reflectivepolarizer and an absorbing polarizer. The transmission spectrum for suchlaminate was measured, and is shown as curve 1401 a.

Visual inspection of the various laminates, including viewing thelaminates in transmission for p-polarized light at a 60 degree polarangle in the y-z plane, yielded conclusions that were consistent withthe observations of the reflective polarizer films themselves. Inparticular, with regard to laminates that used the Example 1 reflectivepolarizer, the laminates exhibited more color than laminates made withthe Comparative Example 3 reflective polarizer, but the amount of colorof such laminates was still at an acceptable level with regard to somedisplay applications.

Second laminates were made that included not only the reflectivepolarizer and the absorbing polarizer, but also a glass layer, asdepicted schematically in FIG. 8. In these second laminates, the glasslayer was the back panel plate of a LC panel taken from an iPad4 devicemade by Apple Computer, Cupertino, Calif. Also, in these secondlaminates: there was no air gap between the absorbing polarizer and thereflective polarizer; the polarizers were oriented such that their passaxes were substantially aligned, and such that their block axes werealso substantially aligned; no light diffusing layer or structure wasdisposed between the reflective polarizer and the absorbing polarizer,and no light diffusing layer was included in the laminate at all.

Visual inspection of the second laminates, including viewing thelaminates in transmission for p-polarized light at a 60 degree polarangle in the y-z plane, and viewing the film in an operating iPad4device with the entire display set to a white output color, yieldedconclusions that were consistent with the observations of the reflectivepolarizer films themselves. In particular, with regard to secondlaminates that used the Example 1 reflective polarizer, the laminatesexhibited more color than second laminates made with the ComparativeExample 3 reflective polarizer, but the amount of color of such secondlaminates was still at an acceptable level with regard to some on-glassdisplay applications.

Following is a non-comprehensive list of embodiments of the presentdisclosure.

-   Item 1 is a laminate, comprising:    -   a reflective polarizer having only one packet of microlayers        that reflects and transmits light by optical interference, the        microlayers configured to define a first pass axis, a first        block axis, and a first thickness axis perpendicular to the        first pass axis and the first block axis; and    -   an absorbing polarizer having a second pass axis and a second        block axis, the absorbing polarizer attached to the reflective        polarizer with no air gap therebetween and such that the first        and second pass axes are substantially aligned;    -   wherein the packet of microlayers includes alternating first and        second microlayers, the first microlayers being biaxially        birefringent;    -   wherein the reflective polarizer in isolation is characterized        by a spectral transmission, for p-polarized light incident at a        polar angle of 60 degrees in a reference plane that includes the        first pass axis and the first thickness axis, in which        -   (a) the spectral transmission has a value in a range from            70% to 90% for at least some wavelengths from 450 to 700 nm;            and        -   (b) a high frequency spectral variability Δ, over a            wavelength range from 400 to 700 nm, is less than 0.08.-   Item 2 is the laminate of item 1, wherein no light diffusing layer    or structure is disposed between the reflective polarizer and the    absorbing polarizer.-   Item 3 is the laminate of item 1, wherein the laminate is devoid of    any light diffusing layer or structure.-   Item 4 is the laminate of item 1, further comprising a light    diffusing layer or structure disposed between the reflective    polarizer and the absorbing polarizer, the light diffusing layer or    structure having a haze of less than 30%.-   Item 5 is the laminate of item 1, wherein the laminate further    comprises a glass layer, the absorbing polarizer disposed within the    laminate between the reflective polarizer and the glass layer.-   Item 6 is the laminate of item 5, wherein the glass layer is part of    a liquid crystal display (LCD) panel.-   Item 7 is the laminate of item 1, wherein the high frequency    spectral variability Δ is a standard deviation of the spectral    transmission relative to a best fit curve of the form    a₀+a₁λ+a₂λ²+a₃λ³.-   Item 8 is the laminate of item 1, wherein Δ, over the wavelength    range from 400 to 700 nm, is less than 0.05.-   Item 9 is the laminate of item 8, wherein Δ, over the wavelength    range from 400 to 700 nm, is at least 0.01.-   Item 10 is the laminate of item 1, wherein the spectral    transmission, for p-polarized light incident in the reference plane    at the 60 degree polar angle, is less than 90% throughout a    wavelength range from 400 to 500 nm.-   Item 11 is the laminate of item 1, wherein the reflective polarizer    is a central portion of a reflective polarizer web.-   Item 12 is the laminate of item 1, wherein the reflective polarizer    is an edge portion of a reflective polarizer web.-   Item 13 is the laminate of item 1, wherein at least some of the    first microlayers have refractive indices nx, ny, nz along    respectively the first block axis, the first pass axis, and the    first thickness axis, and wherein |ny−nz|>0.05, and wherein    |nx−ny|>0.08.-   Item 14 is the laminate of item 1, wherein the reflective polarizer    has a physical thickness of less than 50 microns.-   Item 15 is the laminate of item 14, wherein the physical thickness    of the reflective polarizer is in a range from 20 to 40 microns.-   Item 16 is the laminate of item 1, wherein the laminate consists    essentially of the reflective polarizer, the absorbing polarizer,    and an adhesive layer that bonds the reflective polarizer to the    absorbing polarizer.

Unless otherwise indicated, all numbers expressing quantities,measurement of properties, and so forth used in the specification andclaims are to be understood as being modified by the term “about”.Accordingly, unless indicated to the contrary, the numerical parametersset forth in the specification and claims are approximations that canvary depending on the desired properties sought to be obtained by thoseskilled in the art utilizing the teachings of the present application.Not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof the invention are approximations, to the extent any numerical valuesare set forth in specific examples described herein, they are reportedas precisely as reasonably possible. Any numerical value, however, maywell contain errors associated with testing or measurement limitations.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the spirit and scopeof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein. The readershould assume that features of one disclosed embodiment can also beapplied to all other disclosed embodiments unless otherwise indicated.It should also be understood that all U.S. patents, patent applicationpublications, and other patent and non-patent documents referred toherein are incorporated by reference, to the extent they do notcontradict the foregoing disclosure.

1. A display, comprising: a liquid crystal panel; and a reflectivepolarizer attached to the liquid crystal panel with no air gaptherebetween, the reflective polarizer comprising a coherent grouping ofa plurality of microlayers reflecting and transmitting light by opticalinterference, such that for substantially normally incident light andfor at least one wavelength in a first wavelength range from 450 nm to700 nm, the plurality of microlayers substantially reflects the incidentlight polarized along a block axis and substantially transmits theincident light polarized along an orthogonal pass axis, and such thatfor light incident at 60 degrees and linearly polarized in a plane ofincidence of the incident light and for a second wavelength range from400 nm to 700 nm, the plurality of microlayers: transmits between 70% to90% of the incident light for at least one wavelength in the secondwavelength range; and has a high frequency spectral variability of lessthan 0.08 over the second wavelength range.
 2. The display of claim 1further comprising an absorbing polarizer attached to the reflectivepolarizer with no air gap therebetween, the absorbing polarizer having apass axis substantially aligned with the pass axis of the reflectivepolarizer.
 3. The display of claim 2, wherein the absorbing polarizer isdisposed between the liquid crystal panel and the reflective polarizer.4. The display of claim 1, wherein the coherent grouping of a pluralityof microlayers comprises a plurality of alternating first and secondmicrolayers, the first microlayers being biaxially birefringent, thesecond microlayers being substantially isotropic.
 5. The display ofclaim 1, wherein the reflective polarizer is attached to a glass layerof the liquid crystal panel.
 6. The display of claim 1, wherein the highfrequency spectral variability is a standard deviation of a spectraltransmission of the plurality of microlayers relative to a best fitcurve of the form a₀+a₁λ+a₂λ²+a₃λ³.
 7. The display of claim 1, whereinthe high frequency spectral variability is less than 0.05.
 8. Thedisplay of claim 1, wherein the high frequency spectral variability isat least 0.01.
 9. The display of claim 1, wherein an adhesive layerattaches the reflective polarizer to the liquid crystal panel.